Electrical response of ferroelectric/superconducting/dielectric BaxSr1-xTiO3/YBa2Cu3O7- delta/LaAlO3thin-film multilayer structures

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Abstract
Multilayered structures of ferroelectric and superconducting films deposited on dielectric substrates are being developed for use in low-loss tunable microwave components for satellite and ground-based communications. In this paper, we report on the electrical characterization of BaxSr1-xTiO3 (x=0, 0.08 and 0.10)/YBa2Cu3O7- delta /LaAlO3 thin-film multilayer structures. These structures were formed in situ using a pulsed laser deposition method by the sequential deposition of the YBa2Cu3O7- delta (YBCO) high-temperature superconductor (HTS) and the SiTiO3 (STO) or Ba0.10Sr0.90TiO3 (BST) films on 20 mil (0.5 mm) thick (100) LaAlO3 single-crystal substrates and by using a metal organic deposition method to deposit the Ba0.08Sr0.92TiO3 films. These processes produced YBCO films with thicknesses of 300 nm and ferroelectric films with thicknesses of 250 and 500 nm. The YBCO films showed a transition temperature (Tc) above 89 K after deposition and processing of the ferroelectric layers. Electrical characterization of these structures was performed by measuring the capacitance (C), relative dielectric constant ( epsilon r), and loss tangent (tan delta ) of the ferroelectric film in the temperature range from 300 to 40 K and at electric fields (E) from zero to 2.0*105 V cm-1 For 500 nm and 250 nm thick BST films having the same stoichiometry, epsilon r values of 191 and 126, respectively, were obtained at 300 K, 1.0 MHz, and zero E. Corresponding tan delta values of 0.096 and 0.021 were measured for the 500 nm and 250 nm thick samples, respectively, at the aforementioned temperature, frequency and E. It was observed that the amount of variation of epsilon r as a function of E was closely related to the microstructure of the films. Evidence of conductive pathways in the ferroelectric film as well as of ferroelectric domain 'lock-in' under a d.c. bias is presented.

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